Well, thank you both for your replies. I agree that the koi needs to have more water for a home, and I am actively looking for a new place for him. Unfortunately, I still don't know what the diseases are or how to treat him, but I am now changing the water more frequently and trying my best not to stress him further. Hopefully in time the pH level will become that of the treated tap water, and his diseases will heal over time. Thanks again, and wish me luck!